1. Field of the Invention
The present invention relates to an exhaust gas purification filter. More specifically, the present invention relates to an exhaust gas purification filter suitably used for removing particulate matter contained in exhaust gas discharged from an engine.
In recent years, there has been demanded reduction in fuel consumption of an automobile from the viewpoints of influences on the global environment and resource saving. Therefore, internal combustion engines having good thermal efficiency, such as a diesel engine and a direct gasoline-injection engine tend to be used as a powder source for an automobile.
On the other hand, in these internal combustion engines, soot generation upon combustion is a problem. There is required a countermeasure to inhibit particulate matter such as the aforementioned soot from being released from the viewpoint of the air environment.
As a measure for inhibiting the aforementioned particulate matter from being released in the air, there has been proposed removal of the particulate matter in the exhaust gas by the use of an exhaust gas purification filter. As an example of the exhaust gas purification filter, there is a wall flow type exhaust gas purification filter provided with a honeycomb structure and plugging portions. The honeycomb structure has porous partition walls separating and forming a plurality of cells functioning as exhaust gas passages and extending from one end portion to the other end portion. The plugging portions are disposed so as to alternately plug the one side end portions and the other side end portions of the cells of the aforementioned honeycomb structure.
In such an exhaust gas purification filter, when exhaust gas is allowed to flow into the cells having open end portions on one side of the honeycomb structure, particulate matter in the exhaust gas is trapped by the partition walls when the exhaust gas passes through the partition walls. Then, the purified gas from which the particulate matter is removed flows out from the cells having open end portions on the other side.
However, in the case of disposing such an exhaust gas purification filter in an exhaust system of an engine of an internal combustion engine or the like, increase of pressure loss in the exhaust system is caused. As a result, the decrease of engine output may exceed the allowable range as the engine.
In addition, in a conventional diesel engine filter having a very high discharge concentration of particulate matter in exhaust gas, the particulate matter in the exhaust gas is deposited in layers on the surface of the partition walls of the honeycomb structure constituting the exhaust gas purification filter. The aforementioned “layers of the particulate matter” function as a trapping filter. Hereinbelow, particulate matter may be referred to as “PM” or “particulates”. In addition, the “layers of the particulate matter” may be referred to as “PM layers”. Therefore, in a conventional diesel engine filter, high trapping efficiency of the particulate matter can be maintained. On the other hand, in the case that the concentration of particulate matter in exhaust gas is decreased in comparison with a conventional diesel engine by improvement in combustion in a diesel engine, the formation of the aforementioned PM layers can not be expected, and the trapping efficiency of the exhaust gas purification filter cannot be maintained sufficiently. That is, a conventional diesel engine exhaust gas purification filter is produced on the assumption that the concentration of particulate matter in exhaust gas is high. Therefore, when the concentration of particulate matter in exhaust gas is decreased due to the combustion improvement in a diesel engine itself, it is difficult to sufficiently remove particulate matter by a conventional exhaust gas purification filter even if the amount of the particulate matter is small.
Even in a direct gasoline-injection engine, which has a small amount of particulate matter in comparison with a diesel engine, it is becoming necessary to purify exhaust gas by an exhaust gas purification filter for trapping particulate matter. Even in such an exhaust gas purification filter, formation of the PM layers is not expected like the aforementioned diesel engine having the combustion improved, and the trapping efficiency of the exhaust gas purification filter cannot sufficiently be maintained. In addition, also in a natural gas compression ignition type engine or the like, it has a problem similar to that of the aforementioned direct gasoline-injection engine.
Thus, a direct gasoline-injection engine, a natural gas compression ignition type engine, and an advanced technological diesel engine (e.g., diesel engine having improved combustion) has a small amount of particulate matter in exhaust gas in comparison with a conventional general diesel engine. Therefore, in the direct gasoline-injection engine or the like, the absolute amount of particulate matter which should be trapped by an exhaust gas purification filter is small. However, in a conventional exhaust gas purification filter, the exhaust gas discharged from the aforementioned direct gasoline-injection engine or the like cannot be purified sufficiently.
In addition, in order to improve particulate matter-trapping efficiency, it can be considered to reduce the pore size of the partition walls of the honeycomb structure. However, in the case of simply reducing the pore size of the partition walls, the pressure loss of the exhaust gas purification filter is increased.
2. Description of Related Art
As a means to solve the trade-off problem of raising the trapping efficiency of the particulate matter and suppressing the pressure loss when the particulate matter deposits on the partition walls, the following technique has been proposed. That is, in an exhaust gas purification filter for a conventional diesel engine having a very high particulate matter discharge concentration, a technique of sharpening the pore size distribution of the partition wall has been proposed (see Japanese Patent No. 3272746).
In a conventional diesel engine, since the amount of particulate matter in exhaust gas discharged from the engine is large, the required trapping efficiency of a filter necessary to clear the exhaust gas regulation is set to a high level. For example, in the case of the exhaust gas regulation depending on the number of particulates in exhaust gas, the required trapping efficiency of the filter is 99% or more.
In contrast, in a direct gasoline-injection engine and a natural gas compression ignition type engine (i.e., engines having a reduced particulate matter amount), the amount of particulate matter in exhaust gas discharged from the engine is small in comparison with a conventional diesel engine. Therefore, in a filter used for such an engine, the required trapping efficiency is set to be low in comparison with the case of the aforementioned diesel engine. For example, in the case of an engine having a reduced particulate matter amount, the required trapping efficiency of the filter is about 70%.
As described above, there is a difference in the required trapping efficiency between an exhaust gas purification filter used for an engine having a reduced particulate matter amount and the exhaust gas purification filter developed for a conventional diesel engine (e.g. , a conventional diesel particulate filter). Therefore, there is a difference in the configuration of the filter for performing good purification between an exhaust gas purification filter used for an engine having a reduced particulate matter amount and a conventional diesel particulate filter. For example, the best pore size diameter of the partition wall from the viewpoint of satisfying both the reduction in pressure loss and the improvement in trapping efficiency is different between a conventional diesel engine filter and a filter for an engine having a reduced particulate matter amount. Therefore, there is a problem of impossible application of the technique described in Japanese Patent No. 3272746, which is most suitable for a conventional diesel engine, as it is to an engine having a reduced particulate matter amount.
In a direct gasoline-injection engine, a natural gas compression ignition type engine, and an advanced diesel engine adopting measures such as raising the fuel injection pressure or reducing the diameter of a fuel injection nozzle hole (i.e., a diesel engine having a small amount of particulate matter), particulate matter such as soot is oxidized by the high temperature exhaust gas discharged from the engine and the nitrogen oxide (NOx) contained in the exhaust gas, thereby removing the particulate matter in the exhaust gas. Therefore, in an exhaust gas purification filter used for such an engine, it is important to reduce the pressure loss in a state that no particulate matter deposits on the partition walls. This point is also different from a conventional diesel engine filter, and it is suitably used for an engine having a smaller amount of particulate matter. There is a demand for a development of an exhaust gas purification filter having low pressure loss and appropriate purification performance.